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How to cycle a fish tank
A fishless cycle grows beneficial bacteria before you add livestock. Test ammonia, nitrite, and nitrate until the tank can process a dose of ammonia in a day.
Published 2026-03-12 · Updated 2026-08-15
What cycling actually is
Cycling is growing colonies of bacteria that convert toxic ammonia (from waste and food) into nitrite, then into less-toxic nitrate. Plants and water changes then keep nitrate in a reasonable range. Until those bacteria exist in enough numbers, a new tank is not a home — it is a container of water that can poison fish.
You cannot see the bacteria. You infer they are working from tests. That is why a liquid test kit is not optional gear; it is how you know you are done.
Fishless cycle, step by step
Set up the tank with substrate, hardscape, heater, and filter. Use dechlorinated water. For saltwater, mix marine salt to your target specific gravity and keep temperature in the mid-70s °F.
Add a source of ammonia. Many hobbyists use bottled pure ammonia with no surfactants (read the label) and dose to about 2–4 ppm ammonia. Some use a pinch of fish food and wait for it to decay — slower and smellier, but it works. Follow a single method so your tests mean something.
Test ammonia and nitrite every day or two. You should see ammonia rise, then nitrite rise as ammonia falls, then nitrate rise as nitrite falls. When you can dose ammonia and both ammonia and nitrite read zero within about 24 hours, with nitrate present, the cycle is functionally ready.
Do a large water change before adding fish to bring nitrate down. Add a modest first stocking — not the entire planned community in one afternoon.
Shortcuts and what they do not skip
Used filter media from a healthy tank, bottled nitrifying bacteria from a known brand, and established live rock can shorten a cycle. They do not replace testing. Some products do little; some work. Believe the test kit, not the marketing.
Plants help with ammonia in planted freshwater tanks, but they do not make an uncycled, overstocked aquarium safe. Saltwater live rock is biological filtration, not a reason to skip quarantine or parameter checks.
After the cycle
Bacteria live primarily in the filter media and on surfaces. Do not rinse media under tap water; squeeze it in old tank water if you must clean it. When you upgrade filters, run old and new media together for a few weeks.
A cycled tank can still crash if you overclean, overdose medication that harms bacteria, or add a sudden pile of livestock. Test after big changes. For freshwater beginners, see our setup guide; for stocking limits, see stocking a 20-gallon tank.
Frequently asked questions
How long does a fish tank cycle take?+
Often two to six weeks for a fishless cycle, sometimes longer in cold or sparsely seeded tanks. It is done when you can add a dose of ammonia and see it convert to nitrate with no ammonia or nitrite the next day — not when a calendar says so.
Can I cycle with fish?+
A fish-in cycle exposes animals to ammonia and nitrite. If you already have fish in an uncycled tank, do large, frequent water changes, test daily, and consider bottled bacteria from a reputable source. Prefer fishless whenever you can wait.
Do I need a test kit to cycle?+
Yes. Without ammonia, nitrite, and nitrate tests you are guessing. Liquid kits are easier to read than most strips for this job.
Is cycling different for saltwater?+
The same nitrogen cycle applies. Marine tanks often use live rock or a dedicated biological filter. Still test. Do not add clownfish or coral on day one.